(1) Field of the Invention
The present invention relates to a method and apparatus for reducing the amount of nitrogen oxides (hereinafter referred to as "NOx") exhausted during acceleration of an internal combustion engine adapted to operate at an air/fuel ratio (hereinafter referred to as "A/F") higher than a stoichiometric value.
(2) Description of the Prior Art
Heretofore, an internal combustion engine control system has been proposed in which the A/F is controlled to a value larger than a stoichiometric value, namely, to a lean side, during normal operation and normal acceleration (hereinafter referred to also as "learn burn engine") mainly for the purpose of improving fuel economization. In this system, an acceleration increment correction has been suggested so that the A/F somewhat decreases to the rich side in comparison with that during normal operation in order to improve the drivability during acceleration.
Lean burn engines of this type permit fuel economization, but as shown in FIG. 7 which represents the amount of NOx produced relative to A/F, if an acceleration increment correction to set the acceleration increment ratio at about 40% is performed in a lean burn engine in which the A/F is set at around "22" (region A in the figure), the A/F shifts to around "16" (region B in the figure), namely, an A/F region with a larger amount of NOx generated. As a result, the amount of NOx exhausted during acceleration increases, which may cause environmental pollution.